It is important to optimize the economy of biogas production processes which recover cellulose-containing inlet materials such as paper sludge, or at least partly hydrolyzed cellulose-containing process intermediate streams. Such processes are usually dealing with bulk volumes due to high water contents in the streams. Accordingly, limited capacities at waste treatment in large scale facilities correspond to high setup costs due to volumetrically high water content in the cellulose-containing sludge.
JP S53 98 305 A is a prior art document related to the technical field of the present application.
Cellulose is a substrate that is hard to digest by microorganisms normally found in the flora of biogas reactors. The low digestibility of cellulose as is makes it a poor energy and carbon source, resulting in low yields in terms of biogas production.
Bacterial content of the processed material mixtures (e.g. during aeration in a lidless vessel) are difficult to control against spreading diseases and odor, by allowing growth of unwanted microorganisms causing disease and odor. Thus microbial invasion is a realistic concern as long as aerobic waste disposal methods are to be utilized for paper sludge disposal. Therefore suitable methods should be sought to process such microbial fluids with decreased water content.
Also high water content in such aqueous cellulose-containing mixtures requires big reactor volumes and also low reaction rates due to low concentration of reacting substances. There is a further drawback requiring improvement, that unprocessed cellulose-containing waste streams allow only low rates of biogas release, mainly due to mass transfer limitations related to low flowability. Obtainment of high yields with low process costs is difficult with current biogas production processes which use paper sludge related inlet streams.
Another drawback of using cellulose based materials as biogas raw material is low digestibility of cellulose by biogas producing bacterial consortia. Bacteria that are specialized for methane production are poor digesters of cellulose as is.
Heating and cooling of high water content streams require large amounts of energy. Furthermore, to obtain suitable fluidity, the streams are even diluted with large amounts of fresh water, which correspond to environmentally unfriendly methods, which is also a very important concern. High water content of streams to be aerated also increases aeration costs thereof.
Synthetic oil generation techniques like CatLiq, thermal gasification and pyrolysis require fluid materials pumped into very high pressure vessels. Paper sludge as it is in semi-dry form (from 10% dry matter content and above) forms aggregates and does not behave like a pumpable fluid.
Paper sludge cannot be used as soil amendments or fertilizer directly, due to unbalanced Nitrogen, Phosphorus and Potassium levels and long cellulose chains makes mixing of paper sludge with soil harder.